1. Field of the Invention
The present invention relates to a control mechanism for a telephone answering device employing magnetic tape cassettes, and to a ring detection circuit useful with such a device.
2. Description of the Prior Art
Certain characteristics of magnetic tape cassettes make them particularly advantageous for use in a telephone answering device. For the same length of tape, a cassette requires less space than a reel-to-reel arrangement, thereby enabling size reduction of the device without reducing the number of incoming messages that can be recorded. With a cassette, there is no problem of tape accidentally spilling off the reel. Cassettes are easy to handle and replace. If the user should wish to retain the recorded incoming messages, this can be done by simply substituting a new cassette. The outgoing announcement advantageously is recorded on a reentrant loop tape cassette.
Certain mechanical constraints are imposed by the use of tape cassettes. The tape itself is driven by contact between a capstan and a pinch roller. The tape is then wound about a reel or spool within the cassette that is driven by a spindle. The takeup reel spindle must be appropriately rotated so that the tape being driven from the capstan is wound onto the reel under tension and without slack. Thus a drive arrangement is required which both rotates the capstan at a constant rate and which simultaneously rotates the takeup reel spindle in a manner so as to maintain the necessary tape tension. A further requirement in a tape cassette telephone answering device is to provide a rewind capability for the incoming message tape. This includes disengagement of the capstan and driving of the supply reel at a fairly rapid rate.
In the inventor's U.S. Pat. No. 4,118,602 there is a presented a simplified mechanical control arrangement for a telephone answering device employing a reel-to-reel incoming message record tape and a separate outgoing announcement loop. The mechanism employs two control arms connected by a lost motion linkage. The control arms respectively are configured for forward-rewind selection for the incoming message tape, and for answer-playback mode selection. The mechanical arrangement shown therein is well suited for a reel-to-reel tape mechanism with an independent tape loop drive configuration in which the loop is rim driven from the outgoing message takeup reel. Forward-rewind selection is achieved by a tilting a pivotally mounted tape drive motor. However, such mechanical arrangement does not lend itself to use in a tape cassette telephone answering device, owing to the totally different requirements of the cassettes themselves.
Thus it is an objective of the present invention to provide a simplified control mechanism for use in a telephone answering device that employs magnetic tape cassettes. Another objective is to permit the use of a single motor that is fixedly mounted to the answering device chassis to drive both the outgoing announcement tape cassette and the incoming message record tape cassette. A further objective is to provide mechanism for transferring between forward and rewind drive conditions in the message record tape cassette, while retaining the simplicity of the lost motion linked control arms to change the operational mode of the telephone answering device.
Another problem which arises in a tape cassette telephone answering device results from the accessibility to the user of the cassette record/playback heads. For hum prevention, the metal shields of these heads must be electrically connected to the power source ground in the answering device. The telephone line typically carries a 50 volt dc potential, which is considered a hazardous voltage by Underwriters Laboratories. If there were no dc isolation between the telephone line and the power source ground in the answering device, the user might be subject to a 50 volt shock if he should accidently touch grounded metallic components in the answering device, such as the shields of the record/playback heads.
In a reel-to-reel answering device, such as that disclosed in the inventor's U.S. Pat. Nos. 3,780,226 or 3,818,141 or 3,909,537 this is not a problem. The reason is that the entire reel-to-reel unit can be enclosed in a plastic case, so that the user can never touch the metallic components. In such an answering device, it is acceptable to have a dc path between the telephone line and the unit ground. The user is protected from shock by the insulating plastic houing. However, in a cassette telephone answering device, in which the shields of the record/playback heads are exposed and connected to ground, to avoid the risk of shock, it is necessary to have complete dc isolation of the telephone line from the device power source. Such isolation can be achieved by using ac coupling for the ring detection circuitry and using the contacts of a relay that is closed in response to ring detection to connect a transformer primary directly across the telephone line to accomplish line seizure.
An objective of the present invention is to implement such arrangement in a low-cost telephone answering device in which a single transformer is used both to couple audio to and from the telephone line, and as a coupling device for ring detection.
The problem involved with such dual usage of a single transformer is the following. Audio can be efficiently coupled using a very inexpensive transformer of low inductance. Prior to line seizure, the transformer primary must be coupled to the telephone line by a capacitor which, by typical telephone company standards, can be no larger than one microfarad. If the transformer has a typical low inductance value of 0.3 Henry, its low frequency coupling capability will be very poor. A ring signal, which may consist of a sine wave at 20 Hz, will effectively not be coupled at all into the transformer secondary. Normally this means that the low-cost audio transformer cannot also be used in the ring detection circuit. An objective of the present invention, however, is to provide a circuit which permits such a single low-cost, low inductance audio transformer to be used for the dual purpose of both audio coupling and ring detection. The circuit insures dc isolation between the telephone line and the device power supply ground, and permits use of a relay to connect the transformer primary directly to the telephone line to accomplish line seizure at the beginning of the answering cycle.
In these regards, the present circuit is different from the ring responsive circuit disclosed in the inventor's U.S. Pat. No. 3,909,537. That circuit does not use a line seizure relay, but accomplishes telephone line seizure through a dc path which is common to the dc return or ground of the telephone answering device. As discussed above, this is satisfactory in a reel-to-reel device that can be protectively enclosed in a plastic housing, but is not acceptable in a tape cassette unit in which, while changing the cassettes, the user has access to grounded metallic parts. The present circuit also is different in that although the patented system uses a transformer in the ring detection circuit, audio is not coupled via this transformer. Rather, audio is coupled directly from the telephone line which is seized with a separate inductive load. The present invention allows dual use of the transformer, with telephone line coupling that is dc isolated from the answering device circuitry.